Appliance and method for preparing a froth from a food liquid

ABSTRACT

A method for preparing a froth from a milk-based liquid from an appliance which includes a tank for receiving liquid that is to be frothed, and in which liquid a rotatable stirrer is positioned, a stand associated with the tank, and a system for driving the stirrer. The stirrer drive system and the stirrer include magnetic elements allowing the stirrer to be rotationally driven magnetically in the tank. At least one disturbance member is also provided to break or prevent symmetric circulation of the liquid about the median vertical axis of the tank.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 11/746,457 filedMay 9, 2007, which is a continuation of International applicationPCT/EP2005/011921 filed Nov. 8, 2005, the entire content of each ofwhich is expressly incorporated herein by reference thereto.

BACKGROUND

The present invention relates to an appliance and to a method forpreparing a froth from a food liquid such as milk and is intended, forexample, to be used in the context of the preparation of beverages suchas cappuccinos, milky coffees, café latte and macchiato, hot chocolateand other speciality hot beverages.

Speciality beverages in which at least a portion is made up of froth areincreasingly becoming all the rage. The best-known beverage of this typeis a coffee of the cappuccino type. It comprises a liquid portionconsisting of coffee topped by a layer of frothed milk which, because ofits very much lower density, floats atop the surface of the liquid. Ingeneral, preparing one takes time, manipulation operations and cleaning.

The most customary way of preparing a milk-based froth is to pour thedesired amount of milk into the container, immerse a steam outlet pipefrom a coffee machine in the container, agitating it up and down tointroduce the air needed to form the froth.

A first disadvantage with this method is that it entails having a coffeemachine equipped with a steam outlet, and not all machines are.

Another disadvantage lies in the fact that it is generally not possibleto prepare the froth at the same time as preparing the coffee, unless acomplex and expensive machine with two separate fluid circuits, one forextracting the coffee and the other for producing steam, is available.

Another disadvantage associated with the use of coffee machines with asteam outlet stems from the fact that the system for heating the waterin these machines, most often a thermoblock, has to be kept constantlyswitched on in order to produce the steam without having to wait. Thesemachines therefore consume large amounts of power, which makes them notvery economical to operate. It must also be noted that steam pipes soonbecome scaled up when the water used is hard water and this entailsregular maintenance in order to keep them in good condition.

Another disadvantage with this type of machine stems from the fact thatthe quality of the froth is dependent on the skill of the user whichmeans that these machines do not allow the froth obtained to havereproducible properties and a uniform quality.

Another disadvantage stems from the fact that the pipes in contact withthe milk are difficult and not very practical to clean.

There also exists mechanical stirring appliances which are usuallyintended for domestic use for beating froth from more or less viscousfood products such as eggs, ice, juices or the like. The problem withthese appliances is of several natures which means that they areill-suited to producing froth from a milk-based liquid for making abeverage. One disadvantage, for example, stems from the fact that theseappliances stir a liquid or a paste in the cold state, considerablylimiting their potential use. In addition, milk does not froth asconvincingly when cold or at ambient temperature.

Another disadvantage stems from the fact that these appliances areill-suited to frothing the microbiologically sensitive liquids such asmilk. Regular cleaning of the tank of the appliance needs to beenvisaged in order to remove any solid food residue. In addition,heating the milk has a tendency to increase the extent to which cookedor burnt proteins are deposited on and adhere to the surfaces. Theexisting appliances are not, for the most part, well suited to reducingthe encrustation of this solid residue, making cleaning troublesome.

These appliances also have a stirring and driving mechanism which isfixed and intrudes into the tank, and this presents severaldisadvantages: the removal/refitting time is not insignificant, theyhave a tendency to become soiled more quickly, they entail additionalcost as a result of the multiplicity of components, and the stirringmeans are difficult to clean.

Another disadvantage with these appliances stems from the fact thatfrothing in these appliances is not optimal for reasons associated withthe arrangement of the stirring means. Some appliances have a stirringmeans positioned in the middle of the tank. Such an arrangement is notefficient in quickly converting a volume of milk-based liquid into arich froth.

Other appliances have several stirrers. These arrangements are generallymechanical with gearing means needed to coordinate the rotation of thestirrers, and this makes the system intrusive, and therefore lesshygienic, increases the number of parts and makes cleaning a morepainstaking operation.

U.S. Pat. No. 6,318,247 relates to an appliance for preparing hotbeverages or food with stirring such as hot chocolate, for example. Theappliance comprises a system for driving a stirrer of the magneticeffect type. It does, however, have several disadvantages. Firstly, insuch an appliance, the liquid or froth is stirred coaxially with respectto the central axis of the tank, and this causes a circulation in whichsome layers of liquid or froth, particularly the peripheral layers, arenot stirred as extensively as others, particularly the layers near themiddle, because of the centrifuging effect produced by the stirrer. Sucha circulation is therefore not suited to producing a froth of sufficientquality or to reducing the time needed to produce this froth. Inaddition, the structure of the appliance is not hygienic for treating amilk-based liquid and cleaning is not made any easier by such aconstruction.

Other devices for stirring food products which have more or less thesame disadvantages are described in patent documents WO 2004/043213 A1or DE 196 24 648 A1.

Other stirring systems of the magnetic engagement type are described inU.S. Pat. Nos. 2,932,493, 4,537,332 and 6,712,497 and in German patentapplication DE 1 131 372, but none of these provide solutions suited toproducing a quality froth in a short time and hygienically.

German Utility model DE 89 15 094 relates to a refrigerated pot fordispensing a milk-based beverage. The pot comprises refrigerating meansand forced-convection means in the form of a magnetic rotor. Such adevice is not suited to converting the liquid into a froth but simplykeeps the liquid uniformly at the refrigeration temperature forpreservation.

U.S. Pat. No. 3,356,349 relates to a stirring device comprising a heatedtank, magnetic driving mechanism positioned under the tank for driving ahub located in the middle of the tank. The hub is associated with astirring element which is offset with respect to the hub, causing theelement an oscillatory movement. Such a principle of stirring may createa loss in magnetic coupling because of the high moment of inertiagenerated by the offset position of the stirring means with respect tothe axis of rotation of the driving mechanism. This problem increaseswith increasing speed. Such a stirring device can therefore work to stircertain liquids or other substances at low speed but may proveill-suited to converting liquid into a froth at sufficiently high speed.Incidentally, the document does not mention the conversion of a liquidinto a froth using this device.

Thus, there is a need for a device that overcomes the disadvantages ofthe prior art, and this is now provided by the present invention.

SUMMARY OF THE INVENTION

To alleviate the disadvantages of the prior art, the present inventionproposes an appliance that is particularly suited for the production offroth from a milk-based food liquid, and particularly for producing abeverage. This appliance is of more hygienic design, can be cleanedeasily, and is simple to use. In addition, the appliance is able toproduce a froth having reproducible properties and qualities, and in theshortest possible preparation time. The invention also provides a simplemethod of preparation by limiting the amount of user intervention. Thus,the appliance is economical to use.

To this end, the invention relates to an appliance for preparing frothfrom milk or a milk-based liquid, comprising:

a tank for receiving the liquid that is to be frothed, in which liquid arotatable stirrer is positioned, the tank having a median vertical axisand a bottom;

a stand associated with the tank;

a mechanism for driving the stirrer,

a heating device associated with the tank for heating the liquid in thetank and a controller for activating the heating device during stirring,

an electrical power supply for electrically powering the drivingmechanism;

wherein the driving mechanism and the stirrer are configured in such away that the stirrer is driven by the driving mechanism through amagnetic drive effect in order to generate the froth, without amechanical connection member intruding through the wall of the tank; and

at least one disturbance member is provided to break or preventsymmetric circulation of the liquid about the median vertical axis ofthe tank, wherein the disturbance member comprises a magnetic driverthat produces at least one magnetic field that is offset with respect tothe median vertical axis of the tank and the controller is configured insuch a way as to drive the magnetic driver to thereby magnetically drivethe stirrer at a speed that is sufficiently high to convert the liquidinto the froth as it is being heated.

Another appliance for preparing froth from milk or a milk-based liquidcomprises:

a tank to receive the liquid that is to be frothed, in which liquid arotatable stirrer is positioned, the tank having a bottom and a verticalmedian axis;

a stand associated with the tank;

a system for driving the stirrer, and

a heating device associated with the tank for heating the liquid in thetank and a controller for activating the a heating device duringstirring;

with the stirrer drive system and the stirrer itself comprising magneticelements allowing the stirrer to be rotated magnetically in the tank;

wherein the stirrer drive system and the stirrer are positioned offsetwith respect to the median vertical axis of the tank so as to preventsymmetric circulation of the liquid in the tank about the medianvertical axis of the tank; and

wherein the heating device is distributed in such a way as to heat thebottom of the tank.

Yet another appliance for preparing a froth from milk or a milk-basedliquid comprises:

tank to receive the liquid that is to be frothed, in which liquid arotatable stirrer is positioned;

a stand associated with the tank;

a system for driving the stirrer, with the stirrer configured to bedriven and held magnetically in the tank by the stirrer drive system;and

a cordless frothing unit which is removable with respect to anelectrical power supply unit.

The preferred features of these devices are illustrated herein in thedetailed description.

The invention also relates to a method for preparing a froth from milkor a milk-based food liquid in an appliance according to the invention.This method comprises positioning a stirrer in a tank having internaland external surfaces, a median vertical axis and a bottom and whichcontains the liquid to be frothed, and driving the stirrer by a magneticdrive effect, about an axis of rotation offset with respect to themedian vertical axis of the tank and at a speed that is sufficientlyhigh to convert the liquid into froth while heating the liquid duringthe stirring. The heating may be performed by direct and distributedapplication of the heating elements against the external surface of thetank. Also, the distribution of the heating elements can be arranged toproduce an electrical power density at the heated surface of between 15and 25 watts per cm² with an area of coverage of the heated surface bythe heating elements of at least 40%. Preferably, the stirrer isrotationally driven at a speed of at least 1500 revolutions/minute.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 shows a perspective view of the appliance according to theinvention;

FIG. 2 shows a view in section on A-A of the appliance of FIG. 1;

FIG. 3 shows a perspective view in section on A-A of the appliance ofFIG. 1;

FIG. 4 shows a view in perspective and in section on A-A of part of theappliance, particularly of the tank and of the stand taken in isolation;

FIG. 5 shows the stirrer of the appliance;

FIG. 6 shows the stirrer in section;

FIG. 7 shows the direct a heating device associated with the bottom ofthe tank of the appliance;

FIG. 8 shows a view from above of a variant shape of tank with itsstirrer;

FIG. 9 shows a view from above of a variant tank with reliefs fordisturbing the circulation of the fluid;

FIG. 10 shows a view in cross section on B-B of a variant of the tank ofFIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One improvement in the appliance of the present invention stems from thefact that the driving mechanism and the stirrer are configured in such away that the stirrer is driven, through a magnetic drive effect, by thedriving mechanism without a mechanical connection member intrudingthrough the wall.

One of the principles of the invention is in fact based on the fact thatthe appliance is intended to receive and to process a food liquid whichreadily becomes deposited against the walls of the tank, particularlywhen heated, and so the intended system for driving the stirrer isnon-intrusive and therefore more hygienic and simpler to construct andoperate.

Another characteristic is that a disturbance member is provided to breakor prevent a liquid circulation that is symmetric and more or lesscoaxial about the median vertical axis of the tank. Such a configurationensures that the circulation of fluid in the tank is sufficientlydisturbed thus reducing the time needed to produce the froth and givinga higher froth/liquid ratio. A configuration such as this makes itpossible to obtain results using the non-intrusive system of theinvention, and therefore with just one stirrer, thus greatly simplifyingthe appliance and reducing its cost.

As a preference, a heating device is positioned in association with thetank to heat the liquid in the tank. A heating device allow the liquidand the froth to be heated at the same time as the froth is beingproduced.

According to one embodiment of the invention, the appliance comprises adriving mechanism producing at least one magnetic field that is offsetwith respect to the median vertical axis of the tank. An offset magneticfield such as this thus acts as a disturbance member by driving thestirrer offset from the middle of the tank. Thus, as a result of theoffset position of the stirrer, the layers coaxial to the axis of theliquid, froth or liquid/froth stirrer are stirred through a centrifugingeffect in an asymmetric configuration.

Such an arrangement is non-intrusive which means that no transmissionelement is needed through the tank, in order to connect the drivingmechanism to the stirrer, or to connect or guide the stirrer withrespect to the tank, thus making the appliance more hygienic and easierto clean.

As a preference, a stirrer positioning element is also configured toposition the stirrer, in engagement with the driving mechanism along anaxis of rotation that is offset with respect to the median vertical axisof the tank so that the stirrer, when rotationally driven, producesnon-coaxial circulation of the liquid with respect to the medianvertical axis. A positioning element ensures that the stirrer iscorrectly in place locally at the magnetic field thus defined. Itprevents the stirrer from leaving the field before, during or after thestirring operation. It indicates the position of the stirrer to the userin a simple way and requires no special removal/refitting thereof. Inaddition, when the contents of the tank are poured out, the magneticeffect created between the stirrer and the driving mechanism holds thestirrer in place in the tank.

In one embodiment, the positioning element is a relief formed in thesurface of the tank and has at least a shape that more or lesscomplements the stirrer. The stirrer can thus engage the relief orrecess. Sufficient clearance may be provided to allow the stirrer to berotationally driven without excessive friction. In an alternative,surfaces made of materials of construction having low coefficients offriction may be provided.

Thus, the stirrer is put in place simply by simple engaging of shapeswithout requiring any special tool. The stirrer may be free or connectedby a conventional connector, such as and preferably of the quickcoupling type.

The relief or recess and the bottom of the tank together forms anintegral wall. Thus, there is no need to resort to sealing elements,thus reducing the complexity and rendering the appliance more hygienic.The bottom of the tank is understood to mean the entire surface in thebottom part of the tank and more or less forming the entire width of thetank in this part.

The relief is, for example, provided in the actual wall of the tank asan inwards indentation of the tank therefore producing, on the outside,a recess in this wall. The recess thus formed on the outside of the tankcan therefore serve to house the magnetic drive portion of the drivingmechanism so that the stirrer is held in place in elevation and somedistance from the bottom of the tank, thus improving the stirring effectby comparison with a stirrer position adjacent to the bottom of thetank. As a preference, the stirrer is an annular element which fitsaround a relief in the tank.

In one possible variant, the positioning means may be a recess in theinternal surface of the tank, in which the stirrer is partiallypositioned. In any event, the positioning means thus serves to positionboth along the horizontal axis and along the vertical axis in the tank.It also serves to guide the stirrer and prevents the latter fromaccidentally leaving the magnetic field during the stirring operation.

The stirrer may adopt various configurations. In one of these, thestirrer has an annular shape with a central slot fitting the relief,acting as a positioning means, the relief projecting from the bottom ofthe tank, and has at least one magnetically slaved portion positionednear the periphery of the said central slot. A configuration such asthis is effective because it allows the stirrer easily to be fitted intothe tank. In addition, it allows the stirrer to be kept a certaindistance away from the bottom of the tank, which encourages goodfoaming.

In one embodiment, the driving mechanism comprises a magnetic driveportion which engages on the external side of the tank and inside therelief. The magnetic drive portion thus collaborates with at least themagnetically slaved portion of the stirrer. The magnetic drive portionis thus made to rotate by a spindle associated with an electric motor.Magnetic collaboration between the driving mechanism and the stirrer istherefore closer, reducing the risks of detachment or defective driving,even at high speed.

In one configuration of the stirrer, the latter has a peripheral partpossessing stirring elements. These stirring elements may include one orseveral of the following elements: turns of toroidally-shaped wire,impellers, blades, gratings or needles. Certain elements may prove moreeffective than others according to the nature of the liquid, forexample.

In a preferred embodiment, the tank is cylindrical or more or lesscylindrical. However, in one possible variant of the invention, thedisturbance member for preventing or breaking a coaxial movement of thefluid in the tank comprises a tank the configuration of which isnon-cylindrical with respect to the median vertical axis. The tank mayadopt numerous different shapes without departing from the scope of theinvention.

In one possible embodiment, the disturbance member comprises wallsand/or other obstacles extending in relief into the tank in a mannerthat is offset with respect to the median vertical axis. Thus, theseelements form means which break any purely coaxial circulatory movementof the fluid in the tank. These elements may, for example, be fins orother obstacles extending radially and horizontally and/or verticallytowards the interior of the tank. As a preference, these elements areformed as an integral part or as one piece with the rest of the tank.

In another embodiment, the disturbance member comprises a controllerprogrammed to switch the direction of rotation of the stirrer severaltimes while the liquid is being frothed. Specifically, in a symmetricalconfiguration of rotation of the stirrer in the tank, if just onedirection of rotation is maintained, the contents (liquid and froth)move around the tank concentrically with respect to the stirrer and thevarious layers at the same speed always travel along the same radius andtherefore do not mix to create froth. This has the effect of halting theformation of froth. To guard against this problem, discontinuousstirring is therefore envisaged so that the abovementioned centrifugingeffect is interrupted at regular or irregular intervals. These regularinterruptions in the direction of rotation, which preferably have afrequency of the order of 0.3 to 1 Hz, thus allow the liquid-frothmixture to drop back down and come back into contact with the stirrer inorder to be mixed up again as soon as stirring resumes in the oppositedirection.

According to one aspect of the invention, the tank has, in the partdesigned to contain the liquid, a surface which is without joining lineor connection between two parts and without any line demarcating anorifice. Specifically, a liquid such as milk tends very quickly to soilany gap region by the deposition of solids. This phenomenon isaccentuated if the liquid is heated and the liquid then tends to form asomewhat cooked or burnt layer which is therefore difficult to cleanoff. By avoiding these areas that encourage solids to accumulate thehygienic risks are reduced and cleaning is made easier. Thus, as apreference, the tank comprises a surface for receiving the liquid whichis an integral part. The tank may thus be formed of several elementswelded together but which ultimately form an integral part.

To encourage froth to form while at the same time reducing the stirringtime, a heating device are provided to heat the liquid in the chamberduring stirring. These a heating device are preferably means fordirectly heating the tank. In one embodiment, they are electricallyinsulated resistive elements in contact with at least the bottom of thetank. The elements may be chosen from at least one heating circuit ofthe “thick film” kind in contact with an external surface of the tank,or heating resistive elements of the shielded resistor type in contactwith the surface of the tank. Other heating elements may be induction orinfrared a heating device, or a combination of these means with heatingresistive elements.

In a preferred embodiment, the a heating device comprise a heatingcircuit of the electrically insulated “thick film” kind printed directlyonto the external surface of the bottom of the tank. Such aconfiguration makes it possible to increase the power density againstthe tank surface and therefore reduce the time taken to produce thefroth because the liquid more quickly reaches the optimum temperaturefor frothing, and all this is achieved while at the same time avoidingsticking due to a better distribution of the power.

In order to provide a device that is convenient to use and that does notrequire any particular skill or surveillance of the user, the acontroller is set to automatically cut off the electrical power to the aheating device and to the electrical motor according to a preset scheme.More particularly, the a controller comprise a temperature sensorassociated to the a heating device, wherein the a controller isconfigured to cut off the electrical supply to the a heating device at aset point temperature that is determined so that the liquid is heatedbelow its boiling point and during a preset time. The preset schemestored in the a controller is determined by the nature of the liquid(e.g., milk) and the volume of liquid that can be stored in the tank;i.e., the volume of the tank and the use of an eventual fill mark in thetank to indicate to the user the filling level for the liquid in orderto obtain a volume of froth of optimal quality according to the presetscheme.

The appliance according to the invention may advantageously comprise acordless electrical connection assembly allowing the stand and the tank,on the one hand, to be connected to, on the other hand, an electricalpower supply base support that can be connected to the mains. Thus, thestand and the tank forming part of a foaming unit may be moved aroundfor filling, servicing and cleaning. To do that, the cordless connectionassembly comprises a first connector secured to the bottom of the standand a second connector secured to the power supply base support, the twoconnectors being connected together electrically in order to power theelectrical means of the stand, while the stand is standing on the basesupport.

Thus, according to another aspect, the invention relates to an appliancefor preparing a froth from a milk-based liquid, comprising a tank toreceive the liquid that is to be frothed, in which liquid a rotatablestirrer is positioned, a stand associated with the tank; and a systemfor driving the stirrer. The stirrer is configured to be driven and heldmagnetically in the tank by the stirrer drive system and comprises acordless frothing unit which is removable with respect to an electricalpower supply unit.

Thus, one advantage of such an appliance is associated with the mobilityof the frothing unit which is obtained by the combination of a magneticdrive and of a cordless power supply. Such a combination of means makesthe service of dispensing the froth easier when, for example, the frothis poured into a mug for preparing a cappuccino. In this case, thestirrer is also retained in the tank by a magnetic effect and does notfall out, and need not be removed from the tank while the froth is beingserved. The magnetic elements are also configured (sizing, position,size, etc.) so that the stirrer is held in the tank without the risk offalling out when the tank is inclined, with its opening directeddownward to pour out the froth.

The invention also relates to a method for preparing a froth from amilk-based food liquid in an appliance of the types disclosed herein.The method includes driving the stirrer through a magnetic drive effect,asymmetrically with respect to the median vertical axis of the tank, andwithout a stirrer connecting member intruding through the tank.

As a preference, the method involves heating the liquid in the tankduring stirring. Heating is performed preferably by the direct anddistributed application of heating elements against the external surfaceof the tank. The distribution of the heating elements preferablyproduces a mean electrical power density at the heated surface ofbetween 15 and 25 watts per cm² of the heated area. The area of thetotal heated surface covered by the heating elements is also preferablyat least 40%, preferably between 40 and 60% of this area. A directapplication associated with a distribution of power of the heatingelements, as suggested, makes it possible to obtain a hot froth in thetank more quickly while at the same time reducing the risk of solidresidue sticking to the bottom of the tank. This then yields a frothwhich is stable and hot while at the same time minimizing solid residueagainst the walls of the tank and therefore also making the applianceeasier to clean. Such a method finds a particularly suitable applicationin the preparation of beverages such as cappuccinos in combination withor incorporated into a coffee machine.

An appliance for preparing a milk froth is depicted with reference toFIG. 1. An appliance such as this is used to produce froth from a foodliquid that has the property of emulsifying under the combined effect ofmechanical stirring and heat, for example, a milk product such as freshmilk, pasteurized or UHT milk, skimmed milk, semi-skimmed milk or wholemilk, a milk reconstituted from powder and water or from a milkconcentrate and water or alternatively a mixture of milk with some otheringredient such as coffee.

The appliance 1 according to the invention comprises a cordless foamingunit 2 and an electrical base unit 3 onto which the foaming unit 2 iselectrically connected simply by standing it on the latter. Theelectrical base unit 3 is powered with electrical current from the mainsby a lead 30. The cordless foaming unit 2 comprises, on the side, ahandle 28 so that it can be handled with ease, such as for lifting itup, emptying out its contents after frothing or cleaning it after havingemptied it of its contents. The unit comprises a stand 20 on which thehandle 28 is mounted, the stand being closed by a removable lid 22. Thelatter protects the interior, for example, against dust from theexterior, and protects the exterior from the interior, particularlypreventing any liquid or froth from splashing out when the unit isactivated during the stirring operation.

The appliance will now be described in greater detail with reference toFIGS. 2 and 3. The foaming unit 2 comprises the stand 20 in which a tank21 is mounted. The tank 21 receives the liquid that is to be heated andfoamed. It needs to be sealed and of a capacity suited to the volume offroth to be produced. In general, an appliance such as this is designedto produce milk froth with a volume ranging from 50 to 100 cm³, able toserve milk froth for one to three beverages of the cappuccino type.However, a larger-capacity tank may be envisaged according to the need.Of course, the appliance of the invention may also be used to produce ahot liquid when the stirring operation is eliminated or the means forperforming this operation are deactivated.

On the interior (INT) of the tank is positioned a stirrer 4. Means 5 fordriving the stirrer 4 are, for their part, positioned on the exterior(EXT) of the tank, preferably in the space between the tank and theinterior of the stand 20. The stirrer 4 and the driving mechanism 5 arepositioned relative to one another in such a way as to create forcedstirring of the liquid inside the tank without the use of an intrusivemeans crossing the tank. To do that, the stirrer and its drivingmechanism are of the magnetic type which means that the drivingmechanism 5 produces a rotary magnetic field that drives the stirrer,itself equipped with a means slaved to the magnetic field. The magneticmeans are positioned in an offset manner with respect to the centralaxis “I” of the tank so that the stirrer is made to rotate thuscirculating the liquid or froth in a way which is not coaxial to themedian central axis of the tank. In fact, through such a configuration,the liquid and then the froth tend to circulate more or less producingan asymmetric cone more or less centred on the centre of the stirrer.This makes it possible to avoid the formation of layers of constantspeed which remain circulating at the same radius and therefore do notmix with one another. The layers by contrast do mix with one anotherbecause, on the same radius, the layers are circulating at differentspeeds; in particular, for a given radius, the layers circulate athigher speeds on the side where the stirrer is furthest from the edge ofthe tank and at slower speeds on the side where the stirrer is closestto the edge. This encourages all the liquid or froth to circulate insidethe tank, prevents the stagnation of layers of liquid or froth and thusreduces the areas where there is little or no stirring taking place.

To guarantee controlled rotation of the stirrer in the tank without therisk of the latter moving and therefore avoiding loss of control of themagnetic field, a means 6 for positioning the stirrer is provided in thebottom 210 of the tank. The positioning means is thus positionedeccentrically in the tank but in relation to the position of themagnetic field so as to allow the stirrer to be rotationally driven. Inthe case illustrated, the positioning means 6 is a relief formed in thewall of the tank and around which the stirrer 4 is guided in rotationwithout excessive friction. The positioning means could have otherconfigurations and could, for example, be a recess, into which part ofthe stirrer is then inserted.

One advantage of a non-intrusive driving mechanism is that it ispossible to construct a tank in which the liquid-receiving part, and atleast its bottom, comprises no join line or discontinuous connection orline demarcating orifices or the like able to create areas which presenta potential hygiene problem and/or areas that are difficult to clean orrinse. Thus, the liquid-receiving surface of the tank is preferablyformed as an integral piece. Within the meaning of the invention, weldsforming a continuous seam between two elements for example, are notconsidered to be joining lines or discontinuous connections. Bycontrast, a joint using screws, rivets or a nested fit is considered tobe a join or discontinuous connection liable to pose hygiene problems.In particular, cleaning problems could arise if food solid residue canbecome embedded and settle in interstices of the join. The tank 21consisting of its raised surfaces 211 and its bottom 210, thereforeforms a surface for contact with the liquid that is substantiallysmooth, with no areas encouraging attachment of solid food matter suchas protein or the like. Such a tank can be made from a metal that is agood conductor of heat such as stainless steel, copper or aluminium. Itmay, for example, be moulded as a single piece. In the constructionillustrated in FIG. 4, the tank 21 is connected to the edges 200 of thestand 20 by its top edges 230 by any appropriate connecting means suchas welding, brazing, bonding or clipping. The tank alone may also bemounted removably with respect to the stand so that it can easily beremoved for cleaning in a dishwasher. The lid 22 is connected to thetank as a tight fit via an engagement edge 220 and a deformableelastomeric or plastic gasket 221. Other means for closing the lid 22are possible, such as screw-fastening or elastic clipping.

To drive the stirrer, the driving mechanism 5 are arranged to produce amagnetic field passing through the positioning means 6 which forms therelief 60 on the interior (INT) of the tank. To do that, the drivingmechanism 5 comprises a magnetic drive portion 50 housed in the partforming a recess or indentation 61 when viewed from the exterior (EXT)of the tank. This magnetic drive portion thus comprises one or severalelements 500 made of ferromagnetic material. The magnetic drive portion50 is itself driven in direct rotation by a drive spindle 51 connectedto an electric motor 52 vertically aligned with the positioning means 6.In order to obtain advantageous results in terms of froth production,the rotational speed of the stirrer is controlled so that it reaches atleast 1500 revolutions/min. As a preference, the speed is at least 1650revolutions/min, and more preferably still, between 1800 and 2500 rpm.

As shown by FIGS. 5 and 6, the stirrer 4 according to one embodiment ofthe invention has an annular overall shape allowing it to fit freelyaround the relief-forming positioning means. The relief preferably is inthe shape of a cylinder. The stirrer 4 is thus equipped with a centralslot 40 of a complementary shape and of an inside diameter D slightlylarger than the outside diameter D_(o) of the external surface of therelief 60. The diameters are thus determined one relative to the other,so as to allow rotation without excessive friction or, at least, with aminimum of friction, while at the same time ensuring an effectivemagnetic connection with the driving mechanism. The stirrer comprises atleast one magnetically slaved portion which possesses elements made offerromagnetic material such as a pair of magnets 41, 42 which are housedin a plastic support 43 which may be injection-moulded around themagnets or assembled in several parts 430, 431 by clipping, bonding orany other means. The stirrer possesses an external or peripheral portion44 which comprises the actual stirring elements proper. In theembodiment depicted, these elements are formed by a series of turns 45of wire in the overall shape of a torus. To prevent them from deforming,the turns 45 are held in place by an internal annular reinforcingelement 46. When rotationally driven, such an element produces rapidemulsification of the liquid, particularly milk, especially when thefrothing temperatures are obtained at the same time.

However, other equivalent configurations of the stirring elements mayreplace the one depicted. These may, for example, involve impellers,blades, gratings or needles or yet others.

Thus, when the motor is electrically powered, its spindle rotationallydrives the magnetic drive portion 50 and this creates a rotary localmagnetic field collaborating with the magnets 41, 42 and this as aresult causes the stirrer to rotate about the relief. The rotationalspeed of the stirrer is thus regulated to the rotational speed of themotor spindle.

According to one aspect of the invention, a heating device are providedto heat the tank during stirring. Heating facilitates the production offroth and considerably reduces the stirring time. The froth is alsopreferably served hot, particularly when intended for hot beverages suchas cappuccinos. To do that, in one embodiment of the invention, meansfor direct heating of the tank are provided. A direct heating device isunderstood as meaning means of the heating resistive type or equivalent,electrically insulated but thermally directly associated with thesurface of the tank.

In one embodiment of the invention, the direct-a heating device comprisea heating circuit of the “thick film” type printed directly onto theexternal surface of the tank, preferably onto the external surface ofthe bottom of the tank. The advantage with this technology is that itprovides a good distribution of the power transmitted to the tank (thatis to say, therefore, a greater density in watts per square centimeter).In consequence, the liquid is prevented from sticking to the surface ofthe tank while at the same time ensuring excellent transmission of heatto the liquid.

FIG. 7 illustrates one example of a type of printed heating circuit ofthis type. In one possible embodiment, a circuit extends over just thebottom of the tank (for example, as in the embodiment of FIG. 7). Inanother possible embodiment, a circuit extends over the bottom and theside walls of the tank. In another possible embodiment, several circuitsare provided, at least one of them being on the bottom, and at least oneon the sides of the tank.

The circuit 7 as depicted therefore comprises an insulating layer 70associated with the external surface of the heated surface of the tankand a resistive heating printed track 71 which, in use, faces downwards.The track 71 is arranged in such a way as to produce a total power ofabout 250 to 1200 watts, preferably 400 watts applied to the heatedsurface. Its mean electrical power density with respect to the heatedsurface or circuit 7 is thus between 5 and 30 watts per cm², preferablybetween 15 and 25 watts per cm². The track 71 preferably covers at least40% of the heated surface, and more preferably still covers more than50% thereof, so as to produce an adequate distribution of the totalpower over the heated surface thus preventing the milk from sticking andturning brown on the internal surface of the tank. The mean density iscalculated using the formula: mean density=total electrical power/totalsurface area (7).

Electrical contacts 72, 73, 74 are located at the ends of the tracks toprovide the electrical connections for the circuit. According to oneaspect of the invention, the electrical contacts are three in number,associated with three ends; thus, there is one contact 72 associatedwith neutral, one contact 73 associated with live and finally a thirdcontact 74 connected to the low-voltage electric motor to power thelatter at the required voltage (for example: 8.2 volts in the exampleshown). Such a configuration makes it possible to dispense with atransformer for operating the motor and therefore allows the cost andcomplexity of the system to be reduced. In one possible embodiment, theprinted heating circuit comprises an intermediate metal backing plateseparating the layer of insulation from the bottom of the tank. In thiscase, the backing plate is as thin as possible, preferably 1 mm thick orless. A backing plate separate from the bottom may be needed in anembodiment in which the tank is mounted removably with respect to thestand. However, in an embodiment in which the tank is permanentlymounted, the circuit is preferably printed directly onto the tank.

Other technologies may replace a heating circuit of the “thick film”type, for example shielded resistors. However, this technology is lesspreferred because the transmission of power is not over such a largesurface area (but is generally over a simple generatrix) and requires anadditional operation of brazing the resistor directly against thesurface of the tank. In consequence, in order to transmit the samepower, the resistor has to be overheated and this creates areas wherethe milk tends locally to stick to the surface of the tank. Other aheating device comprise induction heating or infrared heating. Thesemeans may be combined or employed in place of the resistive means.

In one embodiment of the invention, a temperature sensor is provided tomeasure the temperature of the tank. The sensor may be attached to theinsulated contact 75 of the “thick film” circuit 7, preferably near themiddle of the bottom of the tank. The sensor may be of the NTC typeconnected to an electronic controller 8. The sensor may also be a simplesensor of the bimetallic strip type which is less precise and lessrepeatable but does not require associated control electronics. Thecontactors of the heating device are themselves electrically connectedto the controller 8 by blades, wires and/or other means. When thetemperature reaches a given set point temperature, the controller cutsoff the electrical supply circuit to the a heating device. The set pointtemperature is determined such that the liquid is heated to below itsboiling point, preferably in a temperature range that is optimal forfrothing the liquid. In the case of milk, for example, the liquidtemperature is brought to a temperature of between 60 and 80° C. inunder 40 seconds, preferably between 65 and 75° C. in about 30 seconds.Of course, the temperature rise dynamics are partially dependent onfactors such as the volume to be heated and the initial temperature ofthe liquid.

The stand comprises an “on” button 80 that the user can operate. Whenthis button is pressed or activated by any other means it jointlyswitches on the electrical power supply to the a heating device and theelectrical power supply to the electric motor for turning the stirrer.Thus, the liquid is both directly heated and stirred at the same time. Atimer is associated to the a controller to automatically switch theelectric motor off. The running period of the electric motor isprogrammed in the a controller as a function of the liquid to be frothedand its volume. For milk, the running period is fixed within a rangeperiod of from about 30 to 60 seconds. Preferably, the tank comprises afill mark to visually indicate to the user the amount of liquid to befilled in the tank. The fill mark correspond to a predetermined level ofliquid to be frothed and to programmed running period adapted to deliverthe optimum froth. For example, a volume of about 50 ml of milk can thusbe converted into a stable froth at 75° C. in under 30 seconds.

The cordless foaming unit 2 and the electrical base unit 3 are connectedby a collection 9 of general connectors known as “cordless” connectors.A first connector is connected to the central lower part of the stand20. The first connector is electrically connected to the electricalelements and control elements of the foaming unit, particularly thecontroller, the electric motor and the a heating device. A secondconnector of complementary shape is associated with the electrical baseunit which can be connected to the mains by the electric lead.

The first connector 90 comprises electric terminals, typically aneutral, a live and an earth, concentrically arranged and which cancollaborate with electric sockets of the second connector 91. The firstand second connectors are configured in terms of their shapes tocollaborate in engagement with one another simply by standing thefoaming unit vertically (or more or less vertically) on the electricalbase unit so that the electrical terminals (live, neutral, earth) of thefirst connector 90 connect on contact with the respective electricalsockets (live, neutral, earth) of the second connector 91. An example ofa collection of connectors of this type which is known per se isdescribed in detail in U.S. Pat. No. 5,971,810. The advantage of such asystem is that, in the application of the invention, it allows thefoaming unit to be disconnected easily so that the froth can be servedand the tank can be cleaned. It also allows a tank to be designed whichis equipped with a heating device directly associated with the a heatingdevice, for better heating efficiency and therefore a lower frothpreparation time. Other possible known types of cordless connector maybe used as equivalent means without departing from the scope of theinvention.

FIG. 8 depicts an embodiment in which the tank is non-cylindrical withrespect to the vertical axis I. In this example, the tank 21 has anelongate shape with two convex first sides 215, 216 and two second sides217, 218, at 90° to the first sides, which are concave. The stirrer 4and the positioning means 6 will then be positioned at the middle of thetank as illustrated or alternatively may be offset from the middle. Theshape of the tank is such that it produces a disturbed circulation ofthe liquid and the froth thus encouraging the rapid creation of frothand reducing any possible remaining volume of liquid.

FIGS. 9 and 10 illustrate another embodiment of the invention, in whichembodiment the tank 21 has walls and/or obstacles extending as reliefsoffset from the median vertical axis I. In particular, in the exampleillustrated, a plurality of protrusions 212 for disturbing thecirculation of fluid are present in the bottom and likewise 213 on thesides of the tank. These walls run more or less radially and along justsome portions of the tank. They are preferably offset to encourage thefluid in the tank to flow in a sinuous path. It goes without saying thatnumerous different and equivalent arrangements are within the competenceof the person skilled in the art without thereby departing from thescope of the invention.

1. An appliance for preparing a froth from milk or a milk-based liquid,comprising: a tank to receive the liquid that is to be frothed, in whichliquid a rotatable stirrer is positioned, the tank having internal andexternal surfaces, a median vertical axis and a bottom; a standassociated with the tank and having a bottom portion; a system fordriving the stirrer, with the stirrer configured to be driven and heldmagnetically in the tank by the stirrer drive system; and a cordlessfrothing unit which is removable with respect to an electrical powersupply unit.
 2. The appliance of claim 1, wherein the driving systemincludes a positioning element for assisting in positioning the stirreralong an axis of rotation that is offset with respect to the medianvertical axis of the tank so that the stirrer, when rotationally drivenabout the offset axis of rotation, produces non-coaxial circulation ofthe liquid with respect to the median vertical axis.
 3. The appliance ofclaim 2, wherein the positioning element is a relief or a recess formedin the internal surface of the tank and has at least a shape that moreor less complements the stirrer so as to accept the latter in rotationalengagement, wherein the relief or recess and the bottom of the tanktogether form an integral wall.
 4. The appliance of claim 3, wherein thestirrer has an annular shape with a central slot configured to engagewith the relief, the relief projecting from the bottom of the tank intothe interior thereof, with the stirrer having at least one magneticallyslaved portion positioned near the periphery of the central slot.
 5. Theappliance of claim 3, wherein the stirrer drive system comprises amagnetic drive portion which engages the external surface of the tankand inside the relief in order to collaborate at least with themagnetically slaved portion of the stirrer, which magnetic drive portionis made to rotate by a spindle associated with an electric motor.
 6. Theappliance of claim 1, wherein the stirrer has a peripheral partpossessing stirring elements including one or more of turns oftoroidally-shaped wire, impellers, blades, gratings or needles.
 7. Theappliance of claim 1, wherein the cordless frothing unit includes aconnection assembly comprising a first connector secured to the bottomportion of the stand and a second connector secured to a base support ofthe electrical power supply, the two connectors being connected togetherelectrically in order to power the electrical means of the stand, whilethe stand is standing on the base support.
 8. The appliance of claim 1,which further comprises a heating device for heating liquid in the tank,wherein the heating device comprises electrically insulated heatingresistive elements in contact with at least the bottom of the tank. 9.The appliance of claim 8, wherein the heating device consists of atleast one thick film heating circuit in contact with the externalsurface of the tank.
 10. The appliance of claim 8, wherein the heatingdevice includes heating resistive elements of the shielded resistor typein contact with the wall, an induction or infrared heating device, or acombination thereof.
 11. The appliance of claim 8, which furthercomprises a controller that includes a temperature sensor associatedwith the heating device, wherein the controller is configured to cut offpower supplied to the heating device at a set point temperature that isdetermined so that the liquid is heated below its boiling point andduring a preset time.
 12. A method for preparing a froth from milk or amilk-based food liquid, which comprises: positioning a stirrer in a tankhaving internal and external surfaces, a median vertical axis and abottom and which contains the liquid to be frothed, and driving thestirrer by a magnetic drive effect about an axis of rotation that isoffset with respect to the median vertical axis of the tank and at aspeed that is sufficiently high to convert the liquid into froth whileheating the liquid during the stirring to form the froth.
 13. The methodof claim 12, which further comprises rotationally driving the stirrer ata speed of at least 1500 revolutions/minute while controlling activationof the heating during the stirring to reduce stirring time.
 14. Themethod of claim 12, wherein the heating is performed by direct anddistributed application of the heating elements against the externalsurface of the tank.
 15. The method of claim 13, wherein thedistribution of the heating elements produces an electrical powerdensity at the external surface of between 15 and 25 watts per cm² withan area of coverage of the external surface by the heating elements ofat least 40%.
 16. The method of claim 13, which further comprisessensing temperature associated with the heating elements, and cuttingoff the heating at a set point temperature that is determined such thatthe liquid is heated below its boiling point and during a preset time,with the heating elements distributed in such a way as to heat thebottom of the tank.
 17. The method of claim 12, wherein the driving ofthe stirrer provides movement of liquid coaxially to the axis of thetank through a centrifuging effect in an asymmetric configuration whichis non-intrusive along the tank internal surface, thus making the tankmore hygienic and easier to clean.
 18. The method of claim 12, whichfurther comprises configuring the tank without a joining line orconnection between parts and without any line demarcating an orifice inorder to avoid soiling of any gap regions in such lines by thedeposition of solids from the milk or milk-based food liquid to thusreduce hygienic risks and facilitate cleaning of the tank.